1. Field of the Invention
The present invention relates to a derivative with a heteroaromatic ring, and a light-emitting element, a light-emitting device, a lighting device, and an electronic device each using a derivative with a heteroaromatic ring.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence have been extensively conducted. In the basic structure of such a light-emitting element, a layer including a light-emitting substance is interposed between a pair of electrodes. By applying a voltage to this element, light emission can be obtained from the light-emitting substance.
Since this type of light-emitting element is a self-luminous type, it has advantages over a liquid crystal display in that visibility of a pixel is high and that no backlight is needed. Therefore, light-emitting elements are thought to be suitable as flat panel display elements. Further, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight and that response speed is very high.
Further, since this type of light-emitting element can be formed to have a film shape, surface light emission can be easily obtained. This feature is difficult to realize with point light sources typified by a filament lamp and an LED or with linear light sources typified by a fluorescent light. Therefore, such light-emitting elements also have a high utility value as a surface light source that can be applied to lighting or the like.
Light-emitting elements using electroluminescence are broadly classified according to whether their light-emitting substance is an organic compound or an inorganic compound. When an organic compound is used as a light-emitting substance, by application of a voltage to a light-emitting element, electrons and holes are injected from a pair of electrodes into a layer including a light-emitting organic compound, so that a current flows. Accordingly, the carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state, thereby emitting light.
Because of such a mechanism, the light-emitting element is called a current-excitation light-emitting element. Note that an excited state of an organic compound can be of two types: a singlet excited state and a triplet excited state, and luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. Furthermore, it is thought that the ratio of S* to T* in a light-emitting element is statistically 1:3.
At room temperature, a compound that converts a singlet excited state into luminescence (hereinafter referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), not luminescence from the triplet excited state (phosphorescence). Therefore, the internal quantum efficiency (ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is thought to have a theoretical limit of 25% on the basis that S*:T*=1:3.
In contrast, by using a compound that converts a triplet excited state into luminescence (hereinafter referred to as a phosphorescent compound), an internal quantum efficiency of 75% to 100% can theoretically be achieved. That is, emission efficiency can be three to four times as high as that of a fluorescent compound. From such a reason, in order to achieve a light-emitting element with high efficiency, a light-emitting element using a phosphorescent compound has been actively developed recently (e.g., see Non Patent Document 1).
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound as described above, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation, the light-emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix including another substance. In that case, a substance serving as a matrix is referred to as a host material, and a substance that is dispersed in a matrix, such as a phosphorescent compound, is referred to as a guest material.
When a phosphorescent compound is used as a guest material, a host material is needed to have triplet excitation energy (an energy difference between a ground state and a triplet excited state) higher than the phosphorescent compound. It is known that CBP, which is used as a host material in Non-Patent Document 1, has higher triplet excitation energy than a phosphorescent compound which exhibits emission of green to red light, and is widely used as a host material in the phosphorescent compound.
However, although CBP has high triplet excitation energy, it is poor in ability to receive holes or electrons; therefore, there is a problem in that driving voltage of the light-emitting element gets higher. Therefore, a substance that has high triplet excitation energy and also can easily accept or transport both holes and electrons (i.e. a bipolar substance) is required as a host material for a phosphorescent compound.
Furthermore, because singlet excitation energy (an energy difference between a ground state and a singlet excited state) is greater than triplet excitation energy, a material that has high triplet excitation energy will also have high singlet excitation energy. Consequently, a substance that has high triplet excitation energy and a bipolar property is also useful in a light-emitting element formed using a fluorescent compound as a light-emitting substance.
[Reference]
[Non-Patent Document]
    [Non-Patent Document 1]M. A. Baldo, and four others, Applied Physics Letters, vol. 75, No. 1, pp. 4-6, 1999